For Quake 2, the most commonly used clients are R1Q2 and Q2PRO. I recommend starting with R1Q2 because it keeps the same look and feel of the original game but offers major behind-the-scenes improvements. You can get texture packs to make the game look more modern, but I don't like to use them.

A list of all active multiplayer servers can be found at http://q2servers.com/. You can usually find people playing action, ctf, deathmatch, and jump mods.

(Source: I still actively play Q2)